The term ultrahigh molecular weight polyethylene refers to linear polyethylenes obtained by the low pressure process and having viscometrically measured average molecular weights of at least 10.sup.6 g/mol, in particular 2.5.times.10.sup.6 g/mol to more than 10.sup.7 g/mol. The above-mentioned molecular weights are determined from the solution viscosities according to the Margolies equation. The method used for this determination is described, for example, in CZ Chemie Technik 4 (1974), 129 et seq.
UHMW-PE occupies a special position among the polyethylenes. It is distinguished by a number of physical characteristics which make it suitable for a wide range of potential applications. Its high resistance to wear, low coefficient of friction compared with other materials, excellent toughness characteristics, and high thermal stability under load are worthy of particular mention. In addition, it is remarkably resistant to a large number of chemicals. Owing to these particular mechanical, thermal and chemical properties, UHMW-PE is now used as a high quality specialty material in a wide range of applications. Examples are the textile industry, mechanical engineering, the chemical industry, and mining.
There are various known processes for the preparation of this special polyethylene grade. Low pressure polymerization of ethylene using Ziegler catalysts, i.e. mixtures of compounds of subgroups IV to VI of the Periodic Table of Elements (IUPAC version) with organometallic compounds of groups I to III of the Periodic Table, have proven suitable. Among the compounds of the elements of subgroups IV to VI, titanium compounds are the most important. The most frequently used organometallic compounds of groups I to III are alkylaluminums and alkylaluminum halides.
In general the Ziegler catalysts are prepared by reduction of Ti(IV) compounds, such as titanium tetrachloride or titanates, with organoaluminum compounds. This yields Ti(III) compounds, which are frequently isolated, suspended in a suitable medium, and mixed with the activator required for polymerization.
In one proven process for the ethylene with UHMW-PE, oxygen content of less than 5 ppm is polymerized at 30.degree. to 130.degree. C. and a pressure of 0.1 to 10 MPa using catalysts which contain titanium (III) halides and organoaluminum compounds in a molar ratio of 1:0.2 to 1:5. Monohydric or polyhydric aliphatic alcohols are added, in amounts of 2 to 10 mol, based on 1 kg of catalyst, to the reaction mixture during polymerization. Diethylaluminum monochloride is used as the organoaluminum component of the catalyst (DE-C-2,361,508).
In another process, unsaturated polymeric organoaluminum compounds, for example isoprenylaluminum or myrcenylaluminum, are used as the aluminum compound in the catalyst system (DE-C-2,361,052).
As a raw material, UHMW-PE is used predominantly in the form of powder; therefore, the powder morphology and consequently the bulk density are important properties on which its processibility depends. Thus, for example, the properties of the porous moldings which are obtained by sintering pulverulent UHMW-PE, as well as the production of fibers having a high modulus or battery separators containing silica as a filler, are determined substantially by the size and shape of the polymer particles and the width of the molecular weight distribution. However, the morphology of the powder is also important for the production process itself and for storage. Coarse particles having a narrow particle size distribution and high bulk density require less energy for drying, and storage requires less space.